Evolution of the Alpha-Esterase Duplication within the Montana Subphylad of the Virilis Species Group of Drosophila

Previous studies on linkage disequilibrium involving four tightly linked genes that code for the alpha-esterases of Drosophila montana suggest that these loci arose from a primitive esterase gene by gene duplication, followed by tandem duplication (Roberts and Baker 1973). We have examined the esterase variants in the closely related species, lacicola, flavomontana and borealis. These studies reveal that borealis has only a single esterase locus, and flavomontana may have only two loci. Cytological studies, using aceto-orcein staining and Hoechst fluorescence of squashes of ganglion chromosomes, reveal acrocentric Y chromosomes for all six species of the montana phylad, with the exception of borealis, which has the primitive rod-shaped Y chromosome. These studies provide evidence against the hypothesis (Stone, Guest and Wilson 1960) that borealis and flavomontana are derived from montana, but support Throckmorton's (1978) conclusion of the early divergence of the former two species. This phylogenetic relationship supports our contention that the difference in the number of esterase genes with active alleles between borealis and montana is based on an increase in the number of genes coding for the alpha-esterases, rather than the retention in borealis of three genes with null alleles.     

The Genetic Basis of a Species-Specific Character in the Drosophila Virilis Species Group

The genetic basis of the species-specific dorsal abdominal stripe of Drosophila novamexicana was examined. The dorsal stripe is present in D. novamexicana and absent in all other members of the Drosophila virilis species group. Interspecific crosses between D. novamexicana and genetically marked D. virilis revealed that all four of the autosomes (except the tiny dot chromosome, which was not marked) and the sex chromosomes (the X and Y chromosome effects could not be disentangled) showed a significant effect on the width of the dorsal stripe. All the autosomes act approximately additively; only minor interactions were detected among them. No significant maternal effects were found. This means that a minimum of five loci are involved in the character difference between the two species, and this is the maximum number that this technique could discern. These results suggest that, based on the number of factors involved in the character difference, the inheritance of this character should be considered polygenic, but because chromosome 2 (the largest chromosome in the species) contributed over half of the variance toward the character difference, it is best to consider the inheritance oligogenic based on effect. The implications of these findings are discussed in light of the importance of macromutation in speciation and the sex chromosome theory of speciation.     

In Situ Hybridisation Analysis of the X-Linked Genes in the Species of the Virilis Group of Drosophila

When estimating the level of DNA sequence variation within and between populations or when planning QTL analysis, it is essential to know the location of the genes under study. In the present work, five X chromosomal genes, earlier localised in Drosophila virilis and D. littoralis, were mapped by in situ hybridisation on the larval polytene chromosomes of four other virilis group species, D. a. americana, D. flavomontana, D. lacicola and D. montana. Conjugation of X chromosomes of the most interesting species pairs was studied in interspecific hybrids. Three of the marker genes were used as RFLP markers to examine the occurrence of recombination in D. flavomontana and D. montana hybrid females. The gene arrangement of all species studied, appeared to be different at the proximal end of the X chromosome, which prevented normal conjugation along the most part of the X chromosome. The data illustrating the locations of five X chromosomal marker genes are presented for D. a. americana, D. flavomontana, D. lacicola and D. montana.     

The Genetics of Postzygotic Isolation in the Drosophila Virilis Group

In a genetic study of postzygotic reproductive isolation among species of the Drosophila virilis group, we find that the X chromosome has the largest effect on male and female hybrid sterility and inviability. The X alone has a discernible effect on postzygotic isolation between closely related species. Hybridizations involving more distantly related species also show large X-effects, although the autosomes may also play a role. In the only hybridization yet subjected to such analysis, we show that hybrid male and female sterility result from the action of different X-linked loci. Our results accord with genetic studies of other taxa, and support the view that both Haldane's rule (heterogametic F1 sterility or inviability) and the large effect of the X chromosome on reproductive isolation result from the accumulation by natural selection of partially recessive or underdominant mutations. We also describe a method that allows genetic analysis of reproductive isolation between species that produce completely sterile or inviable hybrids. Such species pairs, which represent the final stage of speciation, cannot be analyzed by traditional methods. The X chromosome also plays an important role in postzygotic isolation between these species.     

DNA Sequence Variation at the Period Locus Reveals the History of Species and Speciation Events in the Drosophila Virilis Group

The virilis phylad of the Drosophila virilis group consists of five closely related taxa: D. virilis, D. lummei, D. novamexicana, D. americana americana and D. americana texana. DNA sequences from a 2.1-kb pair portion of the period locus were generated in four to eight individuals from each of the five taxa. We found evidence of recombination and high levels of variation within species. We found no evidence of recent natural selection. Surprisingly there was no evidence of divergence between D. a. americana and D. a. texana, and they collectively appear to have had a large historical effective population size. The ranges of these two taxa overlap in a large hybrid zone that has been delineated in the eastern U.S. on the basis of the geographic pattern of a chromosomal fusion. Also surprisingly, D. novamexicana appears to consist of two distinct groups each with low population size and no gene flow between them.     

黑腹果蝇种组的核型与进化研究

观察了国内黑腹果蝇种组34种果蝇的有丝分裂中期核型,其中首次描述了一些新核型。系统地分析了黑腹果蝇种组8个种亚组之间的核型进化关系及种间亲缘关系。结果是:elegans种亚组的核型为A型;eugracilis、melanogaster和ficusphila种亚组的核型为C型;takahashii和suzukii种亚组的核型为C型和D型;montium种亚组的核型为B、C、C’、D、D’、和E型;ananassae种亚组的核型为F、G和H型。从核型分化的角度可以将黑腹果蝇种组分为5个谱系:elegans,eugracilis-melanogaster-ficusphila,takkahashii-suzukii,montium,ananassae。这与2004年Yang等的观点基本一致,正好从核型进化的角度验证了Yang通过DNA序列分析所得到的结果。差别只在于elegans种亚组,作者把它单独列为一支,认为是祖先种亚组。通过选取同一种果蝇的几个不同地域单雌系的核型分析,结果表明:同一种果蝇的核型存在地域差异。这种差异可能是由于不同生境造成,也可能是本身进化程度的差异,或是两种因素相互作用的结果。     

Molecular Organization of the X Chromosome in Different Species of the Obscura Group of Drosophila

Nine single copy regions located on the X chromosome have been mapped by in situ hybridization in six species of the obscura group of Drosophila. Three Palearctic species, D. subobscura, D. madeirensis and D. guanche, and three Nearctic species, D. pseudoobscura, D. persimilis and D. miranda, have been studied. Eight of the regions include known genes from D. melanogaster (Pgd, zeste, white, cut, vermilion, RNA polymerase II 215, forked and suppressor of forked) and the ninth region (lambda DsubF6) has not yet been characterized. In all six species, as in D. melanogaster, all probes hybridize to a single site. Established chromosomal arm homologies of Muller's element A are only partly supported by present results since two of the probes (Pgd and zeste) hybridize at the proximal end of the XR chromosomal arm in the three Nearctic species. In addition to the centric fusion of Muller's A (= XL) and D (= XR) elements, the metacentric X chromosome of the Nearctic species requires a pericentric inversion to account for this result. Previously proposed homologies of particular chromosomal regions of the A (= X) chromosome in the three species of the D. subobscura cluster and of the XL chromosomal arm in the three species of the D. pseudoobscura cluster are discussed in light of the present results. Location of the studied markers has changed drastically not only since the divergence between the melanogaster and obscura groups but also since the Palearctic and Nearctic species of the obscura group diverged.     

黑腹果蝇种组五种核型的报道

通过传统的敲片、Giemsa染色的方法, 本文首次对果蝇属黑腹果蝇种组的5种果蝇 （D. constricta、D. ohnishii、D. ogumai、D. pseudobaimaii、D. tani）染色体的数目和形态进行了分析报道。分析发现：这5个种具有相同的染色体数目（2n=8）和不同的形态。D. pseudobaimaii和D. tani 为2V,1R,1D型;D. constricta染色为2V,1R,1D型且其点状染色体难以辨认;D. ohnishii和D. ogumai 具有相同形态为2V,2R。另外,还发现核型与亲缘关系之间有一定的对应性。     

A Hybrid Dysgenesis Syndrome in Drosophila Virilis

A new example of ``hybrid dysgenesis' has been demonstrated in the F(1) progeny of crosses between two different strains of Drosophila virilis. The dysgenic traits were observed only in hybrids obtained when wild-type females (of the Batumi strain 9 from Georgia, USSR) were crossed to males from a marker strain (the long-established laboratory strain, strain 160, carrying recessive markers on all its autosomes). The phenomena observed include high frequencies of male and female sterility, male recombination, chromosomal nondisjunction, transmission ratio distortion and the appearance of numerous visible mutations at different loci in the progeny of dysgenic crosses. The sterility demonstrated in the present study is similar to that of P-M dysgenesis in Drosophila melanogaster and apparently results from underdevelopment of the gonads in both sexes, this phenomenon being sensitive to developmental temperature. However, in contrast to the P-M and I-R dysgenic systems in D. melanogaster, in D. virilis the highest level of sterility (95-98%) occurs at 23-25°. Several of the mutations isolated from the progeny of dysgenic crosses (e.g., singed) proved to be unstable and reverted to wild type. We hypothesize that a mobile element (``Ulysses') which we have recently isolated from a dysgenically induced white eye mutation may be responsible for the phenomena observed.     

Molecular Evolution of the Metallothionein Gene Mtn in the Melanogaster Species Group: Results from Drosophila Ananassae

Three distinctly different alleles of the metallothionein gene Mtn have been identified in natural Drosophila melanogaster populations: Mtn(.3), Mtn(1), and Dp(Mtn(1)), where the latter designates a tandem duplication of Mtn(1). In Drosophila simulans, only Mtn(.3)-type alleles have been found. It has been suggested that Mtn(.3) is the ancestral allele and demonstrated that a presumed two-step transition from Mtn(.3) to Mtn(1) to Dp(Mtn(1)) is accompanied by an approximate 5-fold increase in RNA levels. We analyzed the evolutionary genetics of the Mtn locus of Drosophila ananassae, a distant relative of D. melanogaster and D. simulans within the melanogaster species group. The Mtn gene of D. ananassae is most similar to Mtn(.3). (i) it is identical with Mtn(.3) at the amino acid level, but differs from Mtn(1) in its terminal codon; (ii) its 3'' UTR contains a characteristic extra DNA segment of about 50 bp which is present in Mtn(.3), but lacking in Mtn(1); (iii) duplications of Mtn were not found in a worldwide sample of 110 wild D. ananassae chromosomes. However, the intron of the Mtn gene in D. ananassae is only 69 bp long, whereas the length of the Mtn(.3) and Mtn(1) introns is 265 bp; and it lacks a polypyrimidine stretch upstream of the 3'' splice site in contrast to the much greater pyrimidine-richness found in the Mtn(.3) and Mtn(1) introns. A short intron (67 bp) was also identified in a D. pseudoobscura Mtn allele, suggesting that the short intron is the ancestral form and that the transition from the short to the long intron occurred within the melanogaster species group. We discuss the significance of this observation with regard to the recently proposed classification of D. melanogaster introns into two groups: short introns (<90 bp) which tend to lack polypyrimidine stretches, and longer ones which have strong 3'' splice signals similar to mammalian introns. A database search revealed that this length dimorphism is an evolutionarily conserved feature of Drosophila introns; transitions from one size class to the other appear to be rare between closely related species (e.g., within the melanogaster subgroup).     

Evolution of the Transposable Element Mariner in Drosophila Species

The distribution of the transposable element mariner was examined in the genus Drosophila. Among the eight species comprising the melanogaster species subgroup, the element is present in D. mauritiana, D. simulans, D. sechellia, D. yakuba and D. teissieri, but it is absent in D. melanogaster, D. erecta and D. orena. Multiple copies of mariner were sequenced from each species in which the element occurs. The inferred phylogeny of the elements and the pattern of divergence were examined in order to evaluate whether horizontal transfer among species or stochastic loss could better account for the discontinuous distribution of the element among the species. The data suggest that the element was present in the ancestral species before the melanogaster subgroup diverged and was lost in the lineage leading to D. melanogaster and the lineage leading to D. erecta and D. orena. This inference is consistent with the finding that mariner also occurs in members of several other species subgroups within the overall melanogaster species group. Within the melanogaster species subgroup, the average divergence of mariner copies between species was lower than the coding region of the alcohol dehydrogenase (Adh) gene. However, the divergence of mariner elements within species was as great as that observed for Adh. We conclude that the relative sequence homogeneity of mariner elements within species is more likely a result of rapid amplification of a few ancestral elements than of concerted evolution. The mariner element may also have had unequal mutation rates in different lineages.     

Identification of One Intron Loss and Phylogenetic Evolution of Dfak Gene in the Drosophila melanogaster Species Group

Intron loss and its evolutionary significance have been noted in Drosophila. The current study provides another example of intron loss within a single-copy Dfak gene in Drosophila. By using polymerase chain reaction (PCR), we amplified about 1.3 kb fragment spanning intron 5–10, located in the position of Tyr kinase (TyK) domain of Dfak gene from Drosophila melanogaster species group, and observed size difference among the amplified DNA fragments from different species. Further sequencing analysis revealed that D. melanogaster and D. simulans deleted an about 60 bp of DNA fragment relative to other 7 Drosophila species, such as D. elegans, D. ficusphila, D. biarmipes, D. takahashii, D. jambulina, D. prostipennis and D. pseudoobscura, and the deleted fragment located precisely in the position of one intron. The data suggested that intron loss might have occurred in the Dfak gene evolutionary process of D. melanogaster and D. simulans of Drosophila melanogaster species group. In addition, the constructed phylogenetic tree based on the Dfak TyK domains clearly revealed the evolutionary relationships between subgroups of Drosophila melanogaster species group, and the intron loss identified from D. melanogaster and D. simulans provides a unique diagnostic tool for taxonomic classification of the melanogaster subgroup from other group of genus Drosophila.     

Variable Rates of Evolution among Drosophila Opsin Genes

DNA sequences and chromosomal locations of four Drosophila pseudoobscura opsin genes were compared with those from Drosophila melanogaster, to determine factors that influence the evolution of multigene families. Although the opsin proteins perform the same primary functions, the comparisons reveal a wide range of evolutionary rates. Amino acid identities for the opsins range from 90% for Rh2 to more than 95% for Rh1 and Rh4. Variation in the rate of synonymous site substitution is especially striking: the major opsin, encoded by the Rh1 locus, differs at only 26.1% of synonymous sites between D. pseudoobscura and D. melanogaster, while the other opsin loci differ by as much as 39.2% at synonymous sites. Rh3 and Rh4 have similar levels of synonymous nucleotide substitution but significantly different amounts of amino acid replacement. This decoupling of nucleotide substitution and amino acid replacement suggests that different selective pressures are acting on these similar genes. There is significant heterogeneity in base composition and codon usage bias among the opsin genes in both species, but there are no consistent relationships between these factors and the rate of evolution of the opsins. In addition to exhibiting variation in evolutionary rates, the opsin loci in these species reveal rearrangements of chromosome elements.